The present invention relates to a multiple recording system.
A signal known as a MUSE signal is a multiplex sub-Nyquist sampling video signal corresponding to sampling values obtained by performing sampling a plurality of times so as to be mutually interpolated with a predetermined relation in sampling position between continued plural fields of a video signal. The MUSE signal is described in detail in an article entitled "-- MUSE--A Transmission System for High Definition Television", published in NHK TECHNICAL REPORT, Vol. 27, No. 7.
For multirecording a MUSE signal together with another signal, there is known a system in which the frequency of a pilot synchronizing signal in an optical video disk is set to 135 f.sub.H /2 (f.sub.H representing a horizontal scanning frequency) and the pilot signal is multirecorded on the optical video disk together with the MUSE signal. The system is described in detail in an article entitled "Signal Format of High Definition Optical Video Disk" published in ITEJ (the Institute of Television Engineers of Japan) TECHNICAL REPORT, Vol. 10, No. 4, VR 75-8 (May, 1986).
Next, a description will be given as to the signal format of the MUSE signal and interpolation processing performed in an MUSE decoder.
The MUSE signal is formed in such a manner that a chrominance signal in a sampled high-definition television signal is compressed in time and time-divisionally multiplexed on a luminance signal, and the thus-obtained time-division multiplex video signal is subsampled for every four fields to be converted into a narrow-band video signal for transmission. As seen in FIG. 8 showing a waveform of the MUSE signal on a transmission line, the chrominance signal C and the luminance signal Y, which are time-divisionally multiplexed with each other, are transmitted together with a synchronizing signal SYNC. Further, in the MUSE system, sub-Nyquist sampling having a subsampling pattern as shown in FIG. 9 is performed.
FIG. 9 shows the positional relation between sampling points in four continued fields. In the illustrated sampling position relation, a sampling value of the sampling point indicated by a white dot is transmitted in the 4f-th field (f being a natural number), a sampling value of a sampling point indicated by a white square is transmitted in the (4f+1)-th field, a sampling value of a sampling point indicated by a black dot is transmitted in the (4f+2)-th field, and a sampling value of a sampling point indicated by a black square is transmitted in the (4f+3)-th field. In FIG. 9, x represents a sampling point having no sampling value to be transmitted; d, a sampling interval; h, a scanning line interval; i, the horizontal scanning direction; and j, the vertical scanning direction. Further in this drawing, sampling values of sampling points which are encircled by a one-dot chain line and indicated by marks other than x are transmitted on a transmission line with the same timing within a horizontal scanning line.
At the receiving side, the MUSE signal is interpolated to thereby restore the time-division multiplex signal. Then, the luminance signal and the chrominance signal are separated from the time-division multiplex signal, and the chrominance signal is expanded in time to thereby restore the high-definition television signal.
The interpolation methods differ from each other between a still picture and and a moving picture, and between a luminance signal and a chrominance signal. FIG. 10 shows the interpolation method in the case of a still picture reproducing mode of a luminance signal. In FIG. 10, the marks represent the same meanings as those of the marks of FIG. 9.
With a picture sampling value of a sampling point of coordinates (i,j) before interpolation stored in a frame memory of a MUSE decoder and a picture sampling value of the sampling point of coordinates (i,j) after interpolation represented by A and B, respectively, the interpolation method of FIG. 10 is represented by the following expression (1): ##EQU1##
As seen from FIG. 10, the method of obtaining a sampling value B(i,j) of a sampling point of coordinates (i,j) having a sample to be transmitted is different from that of obtaining a sampling value B(i,j) of a sampling point of coordinates (i,j) having no sample to be transmitted. However, a sampling value A(i,j) of a sampling point having no sample to be transmitted, which is indicated by x in the drawing, is zero in the frame memory, and therefore the two foregoing methods can be commonly represented by the expression (1).
As described above, in the MUSE system, the transmitted picture sampling values are rearranged as shown in FIG. 9, and interpolation as shown in FIG. 10 is performed. Therefore, the temporal change of a signal waveform on a transmission line does not coincide with the change of luminance or chrominance depending on the position of a picture element on a reproduced picture scene, and the picture sampling values which are stored in the frame memory adjacent to each other are mutually influenced because of interpolation. In the case where a disturbance component is superimposed on the MUSE signal on the transmission line, in view of the manner in which the disturbance is manifest in the reproduced picture, it is necessary to consider the manner of conversion of the transmitted waveform into the reproduced picture.
In the foregoing conventional system in which the MUSE signal is multirecorded by frequency division on an optical video disk together with a pilot having a frequency of 135 f.sub.H /2. Because of imperfections in the characteristics of the optical modulator used in recording, asymmetry of the pits forming the recording tracks, and the like, the pilot component can be mixed with the MUSE signal. Due to such mixing, the pilot components of sampling points can have levels as shown in FIG. 11. (A calculation method of obtaining the levels of FIG. 11 will be described latter.) In FIG. 11, white dots represent the fact that the pilot component has a positive level, and a black dot represents the fact that the pilot component has a negative level. Further, the diameter of each of the white and black dots corresponds to the level of each of the pilot components.
After interpolation, the levels of the pilot components of the sampling points are changed as shown in FIG. 12. (Also, a calculation method of obtaining the levels of FIG. 12 will be described latter.) In FIG. 12, a sampling point indicated by a white dot appears white on the reproduced picture because the level of the luminance signal is positive, while, on the other hand, a sampling point indicated by a black dot appears black in the picture because the level of the luminance signal is negative. Therefore, in the conventional multirecording system, there is a disadvantage that a stripe-like disturbance is generated in the reproduced picture, as shown in FIG. 12, and as a result the quality of the reproduced picture is deteriorated.